Pelletizing small size seeds facilitates handling and planting. Various coating procedures have been used for this purpose. For example, it is known to coat plant seeds with hydrocolloids which hydrate to form gels, and thereby provide gel coatings around singulated seeds. Such coatings have been used particularly with pregerminated seeds, making it possible to handle and plant the pregerminated seeds without physical damage. This protective action can be of particular importance where the seeds have been germinated to the stage of radicle emergence.
Prior art methods of applying hydrogel coatings to plant seeds are described in Redenbaugh et al., U.S. Pat. No. 4,715,143 and Nelson et al., U.S. Pat. No. 4,780,987. The hydrocolloid, such as sodium alginate, is formed into an aqueous solution, and individual seeds are coated with this solution. The coated seeds are contacted with an aqueous solution of a complexing or cross-linking reagent, such as calcium chloride, which provides calcium ions that react with the alginate to increase viscosity and reduce water solubility.
As described in the cited Redenbaugh et al. patent (cols 4-5), the complexing agent may be mixed with the gel-forming solution and applied to the seeds or other living material to be encapsulated. Alternatively, as described in the paragraph bridging columns 4-5, a vibratory nozzle apparatus may be used to eject gel droplets containing the material to be encapsulated. These droplets are coated with a complexing agent.
More commonly, as described in the Nelson et al. patent (cols 6-7), seeds are dispersed in a gel solution which is added drop-wise to the aqueous complexing reagent. Nelson et al. refers to a mechanized procedure using a vibratory nozzle which ejects seed-containing gel droplets from one source and coats the droplets with a complexing agent from another.
Heretofore, as far as is known, cross-linked hydrogel coatings have not been applied to seeds by first building up a dry coating of hydrogel powder around the individual seeds. This type of coating is known for other purposes, as described for example in Sluis et al. U.S. Pat. No. 4,658,539. As therein disclosed, pregerminated seeds are coated by a pilling procedure for purpose of incorporating a growth retarding agent in the coatings.
The Kelco Division of Merck & Co., sells an alginate product under the name "ALGIN-TEX". Commercially available information concerning this product describes an "ALGIN-TEX System" to form cross-linked alginate matrices. As described in this "ALGIN-TEX" commercial brochure, an alginic polymer consists of units of alginic acid which have carboxyl side chains. The carboxyl groups are reactive with metal ions such as sodium to form sodium alginates. They are also reactive with polyvalent metal ions to form cross-links and thereby increase polymer chain lengths. As polyvalent metal ions are reacted with the alginic acid units, the viscosity of the solution increases because of the greater polymer chain lengths. When more fully cross-linked, insoluble fibers are formed which can precipitate out of solution.